1. Field of the Invention
This invention relates to a gear grinding machine equipped with a threaded grinding wheel for performing grinding, and a rotary dressing device for dressing the threaded grinding wheel, a method for dressing a threaded grinding wheel, and a method for grinding a work.
That is, the present invention is arranged to control the position of the threaded grinding wheel optimally by a numerical control (hereinafter referred to as NC control) function provided in the gear grinding machine so that the wheel pressure angle of the threaded grinding wheel can be easily dressing-modified, and the work can be ground by the dressing-modified threaded grinding wheel.
In further detail, the present invention performs the following processing by effectively using an NC control function provided inherently in a gear grinding machine equipped with a rotary dressing device: That is, the present invention easily carries out dressing capable of progressively changing (gradually increasing or gradually decreasing) the wheel pressure angle of a thread, which is formed spirally on the circumferential surface of the threaded grinding wheel, along a spiral path beginning at the starting point of the thread toward the endpoint of the thread. The work can be ground using the thus dressed threaded grinding wheel.
The term “starting point of the thread” refers to one end of the spiral of the thread formed in a spiral shape. The term “end point of the thread” refers to the other end of the spiral of the thread formed in a spiral shape.
2. Description of the Related Art
A gear grinding machine, in which a gear (workpiece or work) after heat treatment is ground by “a threaded grinding wheel”, a gear grinding tool, to finish the gear, has so far been known. The threaded grinding wheel is an annular grinding wheel having a thread (rack tooth) formed spirally on its outer peripheral surface. Grinding is performed, with the positions in an orthogonal coordinate system (positions on the X-axis, Y-axis and Z-axis) of the threaded grinding wheel, the rotational speed of the threaded grinding wheel, and so on being NC-controlled.
As grinding proceeds, the threaded grinding wheel wears, and its sharpness decreases. Thus, after the threaded grinding wheel has ground many gears continuously, the worn-out threaded grinding wheel needs to be dressed by a dressing device to regenerate a sharp cutting edge.
One of the types of gear grinding machines is a gear grinding machine equipped with a dressing device. The dressing device includes a rotary dressing device provided with a rotationally driven disk-shaped dressing tool. With this rotary dressing device, the disk-shaped dressing tool is kept rotationally driven in a vertical plane, and this dressing tool during rotation is brought into contact with the flanks of the thread of the threaded grinding wheel being rotated, thereby carrying out dressing.
To modify the tooth profile pressure angle of the gear as an object to be ground, the wheel pressure angle of the threaded grinding wheel has to be modified as a precondition. Modification of the wheel pressure angle of the threaded grinding wheel is made by dressing the threaded grinding wheel by the dressing device.
That is,
(1) the threaded grinding wheel is dressed by the dressing device to modify its wheel pressure angle;
(2) the gear is ground by the threaded grinding wheel having the modified wheel pressure angle to finish the gear, whereby the “modified wheel pressure angle” formed in the threaded grinding wheel is transferred to the gear; and
(3) the result is obtained that the tooth profile pressure angle of the gear is modified.
To modify the wheel pressure angle of the threaded grinding wheel, it suffices to turn (turn about a vertical axis (Z-axis)) the disk-shaped dressing tool in contact with the thread of the threaded grinding wheel.
Among gear grinding machines, therefore, are those of the type having a mechanism for turning the rotary dressing device. With the gear grinding machine having such a turning mechanism, an operator manually turns the rotary dressing device (dressing tool) about the Z-axis by use of a block gauge, which is a tool for turning, to modify the wheel pressure angle.
Gears are used to transmit a rotating force, and the engagement of the gears may result in gear noises (noises and vibrations). Thus, crowning (rounding) is applied in the direction of the tooth trace of the gear (helical gear) to smooth contact between the gears for a gentle touch, thereby reducing gear noises.
When crowning is applied in the tooth trace direction of the helical gear by a generating gear grinding machine, the tooth profile changes according to the position of the facewidth. The profile in this case is called a bias tooth flank. With the generating gear grinding machine, a bias-in profile is formed in which the pressure angle on the acute-angle side of the helical gear is large.
The shape of the tooth flank of the helical gear affects gear noises, thus creating need for arbitrary control of the amount of the bias imparted to the tooth flank (namely, need for modification of the bias tooth flank). This need is great, particularly, with tooth flank grinding in the finishing step. However, modification of the bias tooth flank is difficult with the current gear grinding machine.
Under these circumstances, the inventor conducted studies with the expectation that the bias tooth flank of a helical gear would be modified by grinding the helical gear with the use of a threaded grinding wheel. To modify the bias tooth flank of a helical gear by grinding the helical gear with the use of a threaded grinding wheel, it is sufficient that the wheel pressure angle of a thread, which is formed spirally on the circumferential surface of the threaded grinding wheel, progressively changes (gradually increases or gradually decreases) along a spiral path beginning at the starting point of the thread (one end of the spiral of the thread formed in a spiral shape) toward the end point of the thread (the other end of the spiral of the thread formed in a spiral shape)
Thus, it suffices to dress the threaded grinding wheel, thereby “progressively changing (gradually increasing or gradually decreasing) the wheel pressure angle of a thread, which is formed spirally on the circumferential surface of the threaded grinding wheel, along a spiral path beginning at the starting point of the thread toward the end point of the thread.”
As described earlier, to modify the wheel pressure angle of the threaded grinding wheel, it is advisable to turn (turn about the vertical axis (Z-axis)) the disk-shaped dressing tool in contact with the thread of the threaded grinding wheel.
Hence, “in order to dress the threaded grinding wheel, thereby progressively changing (gradually increasing or gradually decreasing) the wheel pressure angle of a thread, which is formed spirally on the circumferential surface of the threaded grinding wheel, along a spiral path beginning at the starting point of the thread toward the end point of the thread”, as mentioned above, it is recommendable to exercise control such that the angle of turning of the dressing tool about the Z-axis is progressively changed (gradually increased or gradually decreased) as the threaded grinding wheel is rotated.
To exercise control such that the angle of turning of the dressing tool about the Z-axis is progressively changed, however, a new function of control for turning about the turning axis has to be introduced into the gear grinding machine. The addition of the new turning control function for turning the dressing tool about the Z-axis to the current gear grinding machine has actually posed severe difficulty.
The present invention has been accomplished in light of the above-described problems. An object of the present invention is to provide a gear grinding machine, a method for dressing a threaded grinding wheel, and a method for grinding a work, which enable a current gear grinding machine equipped with a rotary dressing device to perform the following function without changing the position of a dressing tool: “dressing the threaded grinding wheel, thereby progressively changing (gradually increasing or gradually decreasing) the wheel pressure angle of a thread, which is formed spirally on the circumferential surface of the threaded grinding wheel, along a spiral path beginning at the starting point of the thread toward the end point of the thread”.